Fermiology and electron-phonon coupling in the 2H and 3R polytypes of Nb S2

Abstract

We investigate the electronic structure of the 2H and 3R polytypes of NbS2. The Fermi surfaces measured by angle-resolved photoemission spectroscopy show a remarkable difference in size, reflecting a significantly increased band filling in 3R-Nb1+xS2 compared to 2H-NbS2, which we attribute to the presence of additional interstitial Nb, which act as electron donors. Thus, we find that the stoichiometry, rather than the stacking arrangement, is the most important factor in the difference in electronic and physical properties of the two phases. Our high resolution data on the 2H phase shows kinks in the spectral function that are fingerprints of the electron-phonon coupling. However, the strength of the coupling is found to be much larger for the the sections of bands with Nb 4dx2-y2,xy character than for the Nb 4d3z2-r2. Our results provide an experimental framework for interpreting the two-gap superconductivity and latent charge density wave in 2H-NbS2.